In mining and stone industries, it is in many cases important to fractionate crushed stone and gravel into fractions of stones with different sizes. In most cases, fractionating or screening is done by supplying an unfractionated stream of crushed stone or gravel to a vibrating screen provided with a screening deck including screening holes for allowing stones smaller than the screening holes to pass through the holes.
To achieve a good fractionating or screening result, the stream or layer of crushed stone or gravel should neither be too thick nor too thin. If the stream is too thick, material that should pass through the screening holes tends, in larger amount, to leave the screening deck without being screened, as the material tends to travel on top of the screening deck. If the stream is too thin, the material tends to bounce on the screening deck and likewise not pass through the screening holes.
In the prior art, attempts have been made to overcome the above drawbacks. One solution has been to arrange raisings on the screening deck that extend across the traveling direction of the stream and that covers a part of the width of the screen. These raisings slow down the stream and decrease the bouncing of the material.
Another related prior art is disclosed by U.S. Pat. No. 6,484,885, which discloses a screen with raised ribs, the raised ribs being diagonally arranged relative to the traveling direction of the material. The screen is used in drilling wells to screen solid particles from clay, where the raised ribs prevent the mud from migrating and spreading over the screen, but instead concentrate and gather the solids and make them pass through the screen. U.S. Pat. No. 4,465,592 discloses another screen having diagonally arranged raised ribs for the purpose of concentrating the material on the screening surface.
An object of the invention is to provide a screening arrangement that improves the flow of material on the screening arrangement so that an improved screening result is achieved.
Another object of the invention is to provide a screening arrangement that is flexible in relation to the mixes of material to be screened and still provide an efficient screening.
Yet another object of the invention is to enable efficient screening if the feeding of material to the screening arrangement is reduced or disturb in any way.